The present invention relates to a selective calling receiver of the battery saving type.
Various selective calling receivers of the battery saving type have heretofore been proposed in the art. There have also been proposed a variety of formats for selective calling signals used to call selective calling receivers. However, these proposed formats are basically of the same structure. A format for selective calling signals will be described below with reference to FIGS. 1A, 1B, and 1C which show a POCSAG signal described in STANDARD MESSAGE FORMATS FOR DIGITAL RADIO PAGING, Post Office Code Standardization Advisory Group (POCSAG), Autumn, 1980. As shown in FIG. 1A, a POCSAG signal comprises a preamble signal PA composed of 576 bits of alternate "1" and "0", and a plurality of batch signals B. As shown in FIG. 1B, each of the batch signals B comprises a synchronizing signal SC for synchronizing code words, and 16 code words CW1-CW16. As shown in FIG. 1C, the code word CW1 comprises an information bit IB, a check bit CB, and a parity bit PB. The information bit IB of the code word CW1 represents a selective calling code that is a BCH code converted from a selective calling number when MSB=0, and represents a message code that is a BCH code converted from a message when MSB=1. The remaining code words CW2-CW16 are also of the same arrangement.
A call is recognized by the selective calling receiver as follows: As shown in FIG. 1B, the 16 code words CW1-CW16 are divided into 8 groups G1-G8 each composed of two code words. The selective calling receivers of respective users are registered such that each selective calling receiver belongs to any one of the groups G1-G8. If a selective calling receiver belongs to the group G2, for example, the selective calling receiver recognizes a call by determining whether a selective calling code indicative of a selective calling number assigned to the selective calling receiver is contained in the information bits IB of the code words CW3, CW4 of the group G2. For a selective calling receiver which receives the POCSAG signal to recognize a call, it is necessary to receive only two code words CW3, CW4 of the group G2 to which it belongs, and hence the electric current consumed thereby is reduced by this battery saving action. More specifically, the selective calling receiver turns on its receiver circuit only when the code words CW3, CW4 of the group G2 to which it belongs are transmitted, and keeps the receiver circuit turned off otherwise.
The selective calling receiver receives a message as follows: when a message is transmitted, the 16 code words CW1-CW16 are not divided into groups, and the information bits IB of all the code words CW1-CW16 represent message codes. Therefore, when the selective calling receiver receives a selective calling code indicative of a selective calling number assigned to the selective calling receiver, the selective calling receiver interrupts the battery saving action, and begins operating in a mode for continuously receiving the transmitted signal.
Recently, more and more selective calling receivers employ microprocessors for a wider variety of available functions. In such a selective calling receiver, when the microprocessor processes a message while it is receiving a message, the sensitivity with which the selective calling receiver receives transmitted signals may be lowered due to the operation noise of the microprocessor. Therefore, while the selective calling receiver is receiving a message, the microprocessor is deenergized and a message code produced from the received POCSAG signal is temporarily stored in a buffer memory, or while the microprocessor is operating with a low-frequency clock signal, a message code produced from the received POCSAG signal is temporarily stored in a buffer memory, after which the message is processed by the microprocessor.
According to the format of the POCSAG signal shown in FIGS. 1A through 1C, the message may be of any length. Therefore, when a long message is received, the selective calling receiver needs a buffer memory having a large storage capacity. However, an increase in the storage capacity of the buffer memory poses a serious problem because miniaturization is one of the important goals which the selective calling receivers are to attain.